A major contributor to the very substantial developments taking place in information processing is the miniaturization of semiconductor devices marketed by the electronics industry. In fact, the ability to achieve and maintain a leadership position in the fast developing technology for fabricating smaller and cheaper integrated circuits (IC) can play a key role in the economic well-being of any country. One process by which such IC's are mass produced is lithography. Using this technique, a predetermined pattern can be rapidly replicated on the surface of a semiconductor chip using a beam of radiated particles. A reliable, efficient and compact source of radiation or particles is critical to the economic viability of lithography. This is especially so because of the high capital expenditure required for the sources necessitated by further miniaturization.
A good discussion of lithography is contained in "x-ray Lithography Research; A collection of NRL Contributions", R. R. Whitlock, Editor, Naval Research Laboratory Memorandum report 5731 (1987). As discussed in this report, the crystal-growing industry routinely provides silicon crystals about 5" or more in diameter and once a single-crystal ingot is grown, it is sliced into wafers which are then used for device fabrication. Planar technology consists, for example, in selective introduction of dopant atoms into small precisely predetermined areas of the silicon surface to form regions of p- and n- type material. Dopant atoms can be introduced simultaneously into many separate, small regions of the wafer. Therefore, the use of larger diameter wafers and smaller device dimensions minimizes the process cost per device. The technique for replication of a predetermined pattern on a silicon wafer is referred to as lithography. The pattern may correspond, for example, to an opening for introduction of dopant atoms by diffusion or implantation. Lithography involves the application of a thin film of a radiation-sensitive plastic, called a photoresist, onto the surface of the wafer. The photoresist is then exposed to radiation through a mask bearing the desired feature so as to create a shadow image on the resist.
Current state-of-the-art miniaturization requirements by the IC industry demand the ability to resolve submicron minimum features of patterns to be replicated. However, for mass production of critical, leading-edge circuits for computers, memories, signal processors and other devices, resolutions approaching .ltoreq.0.1 .mu.m will be required. Resolution, therefore, is a determining factor in the quest for greater density. However, as is well known, diffraction provides a fundamental barrier to resolution. If d is the line width of the feature in the mask to be replicated and .lambda. is the wavelength of the radiation, the diffraction angle is .lambda./d, so that if s is the mask-to-resist separation, the blur on the resist is .lambda./(d)s. Thus, to reduce blur, it is necessary to use short wavelength radiation, x-rays, or energetic particle beams.
As far as the resolution is concerned, x-ray radiation or particle beams are satisfactory sources. Of course, other factors such as throughput and yield must be considered in selecting the most appropriate source for lithographic purposes. A direct-write technique with tightly focussed electron or ion beams is frequently used for extremely high-resolution process. The wavelength .lambda. of an electron of momentum p is .lambda.=h/p where h is Planck's constant. Thus, for a 20 keV electron beam, .lambda.=2.5.times.10.sup.2 .ANG.. With such short wavelengths, "computer-controlled particle beams are ideal for making high-quality patterns on masks which are then used for" resist exposure in quantity. However, the main problem with the electron- beam direct-write process for mass production is that it is slow compared to parallel exposure through a mask. In addition, particle beams spread out upon impinging on a resist, and there is also some backscattering and thus possible damage to the mask.
Because x-ray sources have a high throughput compared to direct- write techniques, the use of such x-ray sources as lithography has considerable potential. Three common sources of x-ray radiation that appear to be good candidates for lithography are electron-impact (x-ray tube), high-temperature plasma, and synchrotron radiation. To compare these, the following important characteristics must be considered in making a choice between different sources: emission intensity; efficiency of x-ray generation and usage; spectral character of radiation (lines, continuum, etc.); energy range of emitted photons; source size (important for resolution); emission solid angle (determines collimation and exposure area); pulsed or continuous wave (CW). Further, in comparing the different x-ray sources, the use of material the polymethyl methacrylate (PMMA) will be considered. PMMA is a popular, high-resolution resist suitable for submicron work. To fully expose PMMA one requires 1 J/cm.sup.2 of 12 .ANG. radiation. More generally, the range of useful photon energies is 0.5-1.5 keV (20-8 .ANG.). Photon energy influences resist absorption which, along with the intensity, determines the exposure time. In common with ordinary photography, faster resists than PMMA are found to exhibit poorer resolution.
Turning to the properties of the three common sources of x-ray radiation mentioned above, x-ray tubes were the first source to be used for x-ray lithography. The radiation is uncollimated and is in the form of lines (bound-to-bound transitions) and a continuum (free-to-free transitions in the nuclear coulombic field). x-ray tubes are inefficient, with typically much less than 1% of the electron beam energy being converted into (total) x-ray radiation.
x-ray radiation from plasmas at 10.sup.6 -10.sup.7 .degree. K. is in the form of a line spectrum (bound-to-bound transitions), continue with high-energy cut-off (free-to-bound transitions), and continuum (free-to-free bremsstrahlung). Heating of the plasma is achieved using a discharge, as in a z-pinch, or a high powered laser as described in Nagel et al., Electron. Lett. 14,781 (1978), and the radiation is spread over a large solid angle (2.pi. to 4.pi.). The process is necessarily pulsed with a lifetime of 10-100 ns for discharge heating and 1-10 ns for laser heating. Efficiencies are in the range from 1-10%. Some of the problems associated with high-temperature plasma lithography include the following: contamination of mask and/or resist by debris from the plasma; low repetition rate; thermal response of mask and/or resist to very intense, pulsed heating; and significant shot-to-shot variability of plasma sources.
Patents relating to the use of plasma sources include U.S. Pat. Nos. 3,961,197 (Dawson); 4,184,078 (Nagel et al.); and 4,618,971 (Weiss et al.). The Dawson patent discloses a method for producing coherent secondary x-rays from a plasma device using a laser activated plasma source. A disadvantage of this method is that pulsed x-rays are generated and these x-rays are coherent (it being noted that coherent x-rays are have no advantage over non-coherent x-rays in lithography, the ultimate purpose being to burn a semiconductor, rather than maintain a coherent wavefront.) The Nagel et al. patent discloses a plasma source that produces x-rays via laser excitation; the x-rays produced are coherent x-rays. The Weiss et al. patent discloses an arrangement wherein the plasma is contained in a magnetic field established by an electrical current. This has the advantage providing greater control over shot-to-shot variability but does not solve the other problems mentioned above.
Synchrotron radiation is generated by electrons in synchrotron accelerators and storage rings. It is basically magnetic bremsstrahlung due to the curved motions of the particles in the bending (dipole) magnets. Although the radiation process is efficient, use of the emitted radiation is quite inefficient due to the large fraction that is lost onto the vacuum chamber walls (often about 90%). Because the radiation is highly collimated, .ltoreq.1 mrad for a 1 Gev electron beam, the mask-wafer separation and wafer flatness are less critical than with x-rays from a point-source with highly-diverging rays such as plasma sources. The spectrum is continuous so that filters and/or mirrors must be used to select the desired wavelength band for lithography work.
In existing and proposed storage rings, electrons are typically injected at low energy, .sup.- 150 MeV, and then accelerated to about 1 GeV while the bending magnets are ramped up to about 4-5 T. These arrangements are designed for compactness, with linear dimensions on the order of several meters. Superconducting magnet designs are also available and somewhat more compact, although such designs involve added cost and require additional space for the cryogenic system. Thus far, the most important source of x-ray synchrotron radiation is that provided by a dedicated storage rings to be found in many national laboratories throughout the world. However, it will be appreciated that these machines are extremely expensive and occupy a great deal of space. On the other hand, the commercial storage rings for x-ray lithography are only now becoming available.
Patents relevant to the use of synchrotron radiation include U.S. Pat. Nos. 3,947,687 (Fenstermacher) and 4,803,713 (Fujii). The Fenstermacher patent discloses the use of a collimated x-ray beam to improve efficiency. This is accomplished by a collimator that absorbs undesired x-rays instead of reflecting them via a mirror arrangement as in the prior art. The Fujii patent discloses the use of mirrors to deflect the x-rays to the desired position on the mask.
As described below, the present invention involves the use of an electromagnetic undulator in generating x-rays for lithographic applications. The use of periodic undulators and wigglers to achieve higher brightness (energy radiated per unit bandwidth per unit solid angle), and to modify the spectral character of the storage rings is by now well-established. (See, e.g., Winnick et al., Phys. Today.36,49 (1983).) Due to the complexity and construction costs of electromagnets (conventional or superconducting), recent developments in the fabrication of high-field, rare-earth cobalt permanent magnets have led to their almost universal use as insertion devices in storage rings. Reference is made, e.g., to Halbach, Nucl. Instr. and Meth. 169,1 (1980) and Halbach et al., IEEE Trans. Nucl. Sci. NS -28, 3136 (1981).
Defining the dimensionless magnetic field parameter K=(.vertline.e.vertline.B.sub.o .lambda..sub.o)/(2.pi.mc.sup.2), where e is the charge and m is the rest-mass of an electron, c is the speed of light in vacuum, B.sub.o is the peak magnetic induction and .lambda..sub.o is the period of the planar undulator or wiggler, the wavelength .lambda. of the radiation emitted along the beam direction is given by .lambda.=.lambda..sub.o (1+K.sup.2 /2)!/2.gamma..sup.2 !, where .gamma. is the relativistic mass factor. Typically, .lambda..sub.o ranges over 1-10 cm so that for x-ray radiation in the required range of 8-20 .ANG., electron energies upwards of several GeV are required. On the other hand, lower energy electrons, e.g., at 150 MeV, might be used together with extremely short- period insertion devices. However, to maintain the same magnetic field strength there has to be a corresponding decrease in the gap spacing between opposite poles of the magnet and this implies very thin filamentary electron beams and correspondingly high electron-beam brightness.
Finally, U.S. Pat. No. 4,817,124 (Ketterson et al.) discloses the use of a high frequency laser to produce x-rays via the excitation of a piezoelectric material by microwave radiation. Coherent radiation is generated, and the use of a lasing system to generate x-rays also creates an intrinsic problem with respect to the electron beam quality associated with the lasing action.
From the foregoing, one can see that the electronics industry has an ongoing need for x-ray sources that are compact, energy efficient, and inexpensive. Current x-ray sources suitable for x-ray lithography are generally large, energy inefficient, and very expensive, the synchrotron being an excellent example. One key to the success of x-ray lithography will be the extent to which the technology becomes available to the scientific community generally, rather than to just a few large organizations with unusually large resources.
Much work has been done to develop free electron lasers for the many scientific applications requiring a source of coherent x-rays. Generally, the output wavelength .lambda..sub.x of such a laser relates to the microwave input wavelength .lambda..sub..mu. by: EQU .lambda..sub.x =(.lambda..sub..mu. /4.gamma..sup.2)
Where, again, .gamma..sup.2 =1/(1-v.sup.2 /c.sup.2), v=electron velocity, and c=the speed of light.
From this relation, one would naturally try to increase the x-ray output frequency by increasing electron energy. The generation of coherent electron beams at such high energies, however, makes such lasers prohibitively expensive for general users, and, heretofore, not a potential tool for lithography.
On the other hand, there are relatively compact, efficient, and inexpensive sources of high energy, incoherent, electron beams, for example high the power undulator mentioned above.